[DOC] Refactors documentation for `Forwardable`

[ruby-core:71281] [Misc #11639]

From: Sam Morgan <s_morgan@me.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66873 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/forwardable.rb

@@ -57,10 +57,9 @@
 #
 # == Another example
 #
-# We want to rely on what has come before obviously, but with delegation we can
-# take just the methods we need and even rename them as appropriate.  In many
-# cases this is preferable to inheritance, which gives us the entire old
-# interface, even if much of it isn't needed.
+# You could use Forwardable as an alternative to inheritance, when you don't want
+# to inherit all methods from the superclass. For instance, here is how you might
+# add a range of +Array+ instance methods to a new class +Queue+:
 #
 #   class Queue
 #     extend Forwardable